While it's not exactly Christiania, since 2011 Roskilde has had its own tiny, self-organising alternative community in a graffiti-covered area of semi-derelict buildings southwest of the train station.
Drop a coin in the cup and grab a coffee, chat to folks who use the place as a very low-key social centre, or come at noon for the bargain, organic lunches and free wi-fi. There are board games, occasional wandering guitarists and impromptu events. Just ask.